RealWorld

Now that RealWorld, formerly an implementation trick inside GHC, has
become part of the public interface:
stToIO :: ST RealWorld a -> IO a
(to make stToIO sound), I think it needs a better name. The IO monad
does not in fact give you exclusive access to the world, but to a
region of the heap separate from those used by invocations of runST.
(That's not all IO does, of course, but it's the relevant part for
stToIO.) I propose IORegion.